This invention relates to a process for inhibiting the growth of insoluble solids in liquid fertilizer compositions which are prepared from wet process phosphoric acid, particularly those ammonium phosphate and ammonium polyphosphate fertilizers which are produced by the neutralization, with ammonia, of phosphoric acid.
The most widely used method in the fertilizer industry for producing phosphoric acid is commonly referred to as the "wet process", involving the reaction of phosphate rock with sulfuric acid. After filtering to remove calcium sulfate, the reaction mixtuure comprises a rather impure phosphoric acid, containing a variety of impurities, and usually having a concentration, on a weight basis, in the range of about 30% P.sub.2 O.sub.5 to about 40% P.sub.2 O.sub.5, depending upon the process parameters which are chosen. This acid is normally concentrated prior to shipment, yielding a product which contains about 54% by weight P.sub.2 O.sub.5 (and called merchant grade acid) to about 68-76% by weight P.sub.2 O.sub.5 (and called superphosphoric acid). Unfortunately, the concentration step has the effect of increasing the level of impurities present.
Specific impurities which are present in the phosphoric acids are principally dependent upon the composition of the phosphate rock which was used in the wet process. Calcium, sulfur, iron, aluminum, magnesium and fluorine and other elements are usually present as impurities in varying proportions. Also present are various organic compounds, both derived from organic materials which were contained within the phosphate rock, and from the organic reagents used in the wet process for defoaming, crystal growth and the like. Some of the organic impurities are typically found as insoluble particulate solids, while others are soluble in the phosphoric acid. The organic impurities often cause the more concentrated acids to be very dark in color, frequently even black.
Liquid mixed fertilizers have become very popular in the industry, both from the standpoint of the producer and that of the consumer. The major advantages over standard dry mixed fertilizers include elimination of the expensive steps for evaporating water and packaging, as well as the greatly simplified soil application procedures with liquids. In addition, the liquids are easier to handle and transport, since no segregation or caking problems are encountered.
However, serious problems have been experienced in the handling and storage of liquid fertilizers obtained from the ammoniation of phosphoric acids which contain all of their phosphorus values as the acyclic orthophosphate species, since voluminous precipitates of the metallic impurities in the acid are also formed by ammoniation. These precipitates are very difficult to remove by filtration and render the fertilizer composition unsuitable for soil application by means of the usual spray nozzles.
In response to this problem, several workers discovered that by properly concentrating the wet process phosphoric acid to a P.sub.2 O.sub.5 analysis of 60-76%, there are formed acyclic polymers of phosphoric acid, including pyro-, tripoly-, and higher polyphosphoric acids. The presence of a substantial amount of these polymers was found to sequester the iron and aluminum present in the phosphoric acid, and thereby avoid the formation of the voluminous precipitates which otherwise form upon ammoniation. Examples of this are found in U.S. Pat. No. 3,192,013 to Young and U.S. Pat. No. 3,317,306 to Getsinger, both incorporated herein by reference.
A problem relating to the long-term storage of ammoniated phosphoric acids still exists, in spite of the discovery noted above, particularly for fertilizer compositions which were prepared from acids containing high amounts of magnesium (greater than about 0.5% by weight MgO). This results from the insolubility of complex magnesium-containing compounds, such as Mg(NH.sub.4).sub.2 P.sub.2 O.sub.7.4H.sub.2 O and MgAl(NH.sub.4).sub.5 (P.sub.2 O.sub.7).sub.2 F.sub.2.6H.sub.2 O. Typically, the precipitation of magnesium compounds does not become a problem until several weeks or months after the ammoniation preparation of the fertilizer solution. The solids are a nuisance in that they form a scale which is difficult to remove from storage vessels and they can plug spray nozzles in field distribution equipment. They also represent a considerable loss of valuable phosphorus from the fertilizers.
Examples of processes for the removal of magnesium and other metallic impurities from phosphoric acid are those of Mills, as shown in U.S. Pat. No. 4,136,199 and U.S. patent application Ser. No. 954,647, filed Oct. 25, 1978, both of which are incorporated herein by reference. These involve a precipitation of a complex salt-containing fluoride, magnesium and other metals such as sodium and aluminum, initiated by the addition of a substance containing calcium fluoride to heated phosphoric acid, maintaining the elevated temperature for a sufficient period of time, and forming a removable precipitate, thereby lowering the soluble impurity content of the acid.
A precipitation of magnesium from phosphoric acid using fluosilicate has been described in U.S. Pat. No. 3,819,810.
Other workers, including Williams, et al. in U.S. Pat. No. 3,694,153, have used solvent extraction processes to remove metallic impurities such as calcium, magnesium, iron and aluminum, to give a more easily stored fertilizer solution upon ammoniation.
British Pat. No. 1,139,192 shows the use of substituted phosphonic acids, of which amino-tris (methylene phosphonic) acid is exemplary, for inhibiting the growth of precipitates in ammoniated phosphoric acids, particularly when potash and trace metallic elements are also added.
Methods are known for decolorizing wet process phosphoric acids, primarily by removal of the carbonaceous matter, including those of U.S. Pat. Nos. 3,619,161 to Knarr et al (addition of water-insoluble organic agents, removing carbon in the organic layer), 3,630,711 to Burkert (flocculation of carbonaceous material with an aliphatic organic amine, removal as a froth), and 3,969,483 to Stinson et al. (flocculation with aliphatic amines and quaternary ammonium chloride, flotation of carbon), plus the U.S. Defensive Publication Nos. T866,034 of Burch (use of phosphate rock as a settling aid for carbonaceous material) and T942,007 of Stinson et al. (flocculation with primary amines and quaternary ammonium chlorides).
The methods, described above, which involve a precipitation or similar separation step usually suffer from the disadvantage of requiring a considerable period of time, ordinarily without agitation, for the desired phase separation to occur. As a result, it is necessary to provide a plurality of very large, often heated, storage vessels in which the separation may be effected. Similarly, extraction purification processes have the inherent disadvantage of requiring costly equipment and reagents.